This invention relates generally to improvements in devices and methods that are used to test the rheology of fluids, especially ones containing or mixed with particulate material (subsequently referred to simply as “particulate”). The particular field of use is the oil and gas industry.
Various types of fluids are used in the oil and gas industry. Non-limiting examples include drilling muds, cement, gravel slurries, and stimulation treating fluids. Such fluids are typically pumped into oil or gas wells in known manners. It is desirable to know various characteristics of the fluids to determine how such fluids will act upon being pumped and placed in, or circulated through, the wells.
Rheology is the branch of physics dealing with the deformation and flow of matter. Viscosity, elasticity, and consistency are rheological characteristics that sometimes need to be measured for a given fluid. Such rheological characteristics can be non-linear functions of time, temperature, and pressure. Known devices used to test fluids for these characteristics include viscometers, rheometers, and consistometers.
Some fluids used in oil or gas wells carry particulate, and it is typically desired that such fluids support the particulate in suspension for at least some period of time. That is, the particulate is preferably dispersed throughout the volume of a particular fluid during at least part of the time the fluid is used in a well. For example, a fracturing fluid might include a base fluid that can be crosslinked to a gel form so that it can better support a quantity of particulate referred to as a propping agent or proppant. An example of a propping agent or proppant is sand. The fracturing fluid is a fluid that preferably initially increases in viscosity as the fluid approaches the well's treatment zone, to suspend or support the proppant in the fluid during the time the mixture is pumped into a well. The fracturing fluid later “breaks” or decreases in viscosity so that it can easily flow back out of the well, while leaving a clean proppant pack in the fracture. The pumping is under pressure sufficient for the fluid to hydraulically fracture a selected zone of the earth traversed by the well. After fracturing, the fluid may be flushed out with the flow of hydrocarbons from the fractured zone, but the propping agent preferably remains in place to prop the fractures open.
Proppant transport is a function of: (1) wellbore and fracture geometry; (2) volumetric rate; (3) proppant size, concentration, and specific density; and (4) carrier-fluid rheology.
A typical fluid used to transport particulate has a viscosity that changes during the time the fluid is used in a well. Viscosity is defined as the ratio of shear stress to shear rate (velocity gradient). If this ratio changes with shear rate, this may be referred to as “apparent viscosity function.” Viscosity is one parameter of the fluid that defines the fluid's ability to support the particulate in suspension. However, to measure a single viscosity point or the apparent viscosity function does not directly indicate the time during which the fluid will support particulate in suspension and the time during which the fluid will not. That is, a measurement that merely shows a changing viscosity does not indicate when the particulate is in suspension within the fluid and when it is not (i.e., when the particulate has settled out of the fluid).
Elasticity is another parameter of the fluid that defines the fluid's ability to support the particulate in suspension.
Instruments such as Fann Model 50 viscometers are available for measuring viscosity, including at high temperatures and pressures, but elasticity is much more elusive to measure. Additionally, most viscometers, such as the Fann Model 50, are designed only to handle the “clean fluid systems,” e.g., without proppant. It has generally been assumed that higher viscosities will do a better job of transporting proppant, as well as generating the desired fracture geometry.
Several rheological properties directly impact a fracturing fluid's performance: (1) apparent viscosity function, (2) yield stress, (3) dynamic viscosity, (4) rheomalaxis (irreversible thixotrophy), (5) viscoelasticity (for example G′, G″, tangent delta), and (6) the related issue of turbulent-drag reduction. In laboratory research, sample volume is often very limited, thus necessitating rheological testing and evaluation of small quantities. Also, most bench-top rheometers use batch mode, that is, small samples are placed in a testing chamber as opposed to flow-through testing, as is the case for pipe viscometers. This presents the challenge of simultaneously: (a) Imparting viscometric shear history that simulates the wellbore travel path; (b) Not exceeding the proper mechanical energy input—the bench-top batch process should impart about the same amount of integrated work as the wellbore path; and (c) Maintaining satisfactory thermal balance, e.g., being sure not to create localized “hot spots” in the bench-top process because of its batch mode of operation.
In the case of most polymer-based fracturing fluids, the capability to transport proppant is directly related to their “rheological equations of state (RES).” Extent of crosslinking, breaking, shear history, and volume-average shear rate are major factors affecting a fluid's RES.
Where viscous drag dominates, as in the classical case of a tow-winged vertical fracture with parallel-plate geometry, the challenge in proppant transport is to ensure that vertical settling time is much greater than horizontal travel time. Sufficient vertical settling time allows the particle to reach a maximum horizontal distance, thus avoiding a duning effect. Preferably, the horizontal transport time is at least 50 times greater than the vertical settling time.
In the case of crosslinked gels, the elastic forces are designed to dominate, preventing any substantial viscous settling during the fracturing and placement of proppant. It is believed that a minimum value of G′ (oscillatory elastic storage modulus) of 10 to 12 Pa's is sufficient for most 20/40 frac sands.
The complex viscoelastic nature of crosslink fracturing fluids presents a dilemma for the fracturing rheologist. Conventional rheometers are designed for measuring viscoelastic properties through well-controlled oscillatory deformations that are small, non-destructive, and within the linear elastic range. However, the actual fracturing process involves large amounts of shear strain of multiple orders of magnitude, well beyond the linear elastic range. As the breaker reaction begins to dominate, the transport mechanism shifts from elastic to viscous, leading to settling caused by the low viscosity of the broken fluid system.
The Fann Model 50 viscometer was designed for characterizing fracturing gels under simulated downhole temperature-time conditions. However, the Model 50 and most other bench-top viscometers/rheometers are not adequately equipped to handle proppant-laden fluids. In the case of concentric cylinders, the centrifugal forces tend to stratify the particles, thus resulting in nonrelevant data. In cone-plate and plate-plate viscometers, the small gaps necessary to provide torque sensitivities result in “particle jamming.” Additionally, the large density differences between most proppants and conventional fracturing fluids can result in settling during testing, thus producing unreliable results.
U.S. Pat. No. 6,782,735 issued Aug. 31, 2004 and entitled Testing Device and Method for Viscosified Fluid Containing Particulate Material, which is incorporated herein by reference for all purposes, discloses a device and a method for testing a viscosified fluid containing particulate that indicate when the particulate is in suspension within the fluid and when it is not. The device and method stir the fluid and particulate mixture for a time during which the viscosity of the fluid changes such that during a first period of the stirring time substantially all the particulate remains suspended in the fluid and during a second period of the stirring time substantially all the particulate settles out of suspension in the fluid. A signal is generated during the first and second periods such that the signal has a characteristic that changes from the first period to the second period to indicate the change in particle carrying ability of the fluid. Other characteristics, including crosslinking time, can also be determined. A test chamber includes interacting projections extending from the inner surface of a cup receiving the fluid and from an axial support extending into the fluid in the cup. U.S. Pat. No. 6,782,735, Abstract. For example, a viscometer according to U.S. Pat. No. 6,782,735 can be uniquely designed to keep highly concentrated dense particles suspended in fluids while measuring “volume-averaged shear stresses and shear rates.” However, the devices and methods disclosed in U.S. Pat. No. 6,782,735 must be loaded with pre-crosslinked fracturing fluid along with the particulate and breakers such that the proppant is suspended during mounting and testing.
In addition, well fracture fluids are sometimes blended at the well site with the proppant while the fluids are in the “un-crosslinked” state and are at the same time mixed or contacted with chemicals called “crosslinkers” that cause crosslinking after a specified temperature-time history. Un-crosslinked well fluids are not designed to support proppant and attempts to test with conventional equipment results in errors due to proppant settling or fallout. For example, conventional viscometers and rheometers, are not capable of accepting fluids with particulates the size of proppants, ranging from a few hundred microns to 1,000 microns. Usually the clearances between the surfaces used to establish a known shear rate, are on the same order of magnitude as proppants. In the cases in which “large gap devices” have been employed to solve this problem, centrifugal forces and local vortices cause significant error due to sample stratification of the proppant. On the other hand, conventional testing devices that provide constant agitation do not prevent the particle settling, and may operate in the turbulent regime.
Accordingly, there is a need for a device and method to measure the viscous and elastic properties of mixtures of un-crosslinked well fluids, both with and without particulate, including before, during and after crosslinking and before, during, and after breaking, under dynamic conditions at elevated temperatures and pressures at a variety of shear rates and in such a way as to directly indicate particle transport, suspension, and settling.
At least one embodiment of such a testing device and method preferably should also be suitable for use at a well site to properly measure the rheological properties of fluid mixtures before, during, and after crosslinking and breaking.